1. Field of the Invention
This invention relates to a velocity feedback circuit for a position servomechanism, and more particularly to a velocity feedback circuit suitable for application to a position servomechanism in which the position of a controlled object is detected by quantization of the position.
2. Description of the Prior Art
Addition of a velocity feedback circuit to a position servomechanism has been proposed as one of methods for compensating the operating characteristic of the position servomechanism. One form of the velocity feedback circuit widely used in this field hitherto has generally been so constructed that a tachometer generator for detecting the velocity of a controlled object is provided in combination with a position detector detecting the position of the controlled object, and the output from the position detector is fed back to find the error between the detected position and the desired position, the output from the tachometer generator being then subtracted from the value of the position error. In this connection, it has been the recent tendency that a position detector such as a position encoder quantizing the position of a controlled object is frequently used as the position detector for the position servomechanism.
In a position servomechanism provided with a velocity feedback circuit of the kind above described, it is necessary to install the tachometer generator in the control part of the position servomechanism. However, it is generally difficult to additionally install the tachometer generator in the control part of the position servomechanism provided already with the position detector, due to the lack of available space from the dimensional aspect. Therefore, it is not desirable to add the velocity feedback circuit of the above kind to the position servomechanism when the object is to improve the operating characteristic of the position servomechanism mounted on an apparatus.
As another example of the velocity feedback circuit for a position servomechanism using a quantizing position detector, it has been proposed to use, in place of the tachometer generator, a frequency-to-voltage converter (which will be referred to hereinafter as an FV converter) which converts a frequency into a voltage utilizing the output pulse generated at the end of each quantizing period of the output signal from the position detector thereby obtaining a voltage signal proportional to the velocity. Thus, the FV converter is essentially required in the latter example of the velocity feedback circuit although the provision of the tachometer generator is unnecessary.
In each of the two examples described above, the velocity is detected in the form of an analog quantity. Therefore, the velocity feedback circuit is so constructed that the position error is also converted into an analog quantity, and an analog circuit executes subtraction of the velocity from the position error. However, an analog circuit is disadvantageous in that it is susceptible to variations in the power supply voltage and variations in the characteristics of its circuit elements and that many steps of troublesome adjustment are required. Therefore, attention is now being increasingly directed to a so-called digital position servomechanism in which the velocity in digital quantity is subtracted from the position error also in digital quantity, and an amplifier disposed in the stage preceding the controlled object is actuated in response to the result of subtraction. A digital velocity feedback circuit, which is suitable for application to such a digital position servomechanism and detects the velocity in the form of a digital quantity, has been proposed and is disclosed in, for example, Japanese Patent Application Laid-open No. 19880/77. The disclosed digital velocity feedback circuit is so constructed that the value of displacement of a controlled object during a predetermined time is calculated at intervals of the predetermined time, and the calculated displacement value is multiplied by the reciprocal of the predetermined time described above so as to calculate the velocity to be fed back. However, such a circuit requires, for the velocity detection, a relatively long time equal to the duration corresponding to the movement of the controlled object over a distance of several units of quantized displacement. Consequently, the disclosed digital velocity feedback circuit is defective in that such a manner of velocity detection results in a large delay time, and the dynamic characteristic of the position servomechanism is not fully improved.